Sibley's volcano, pushed over on its side, has had a second life as a quarry dug by Kaiser Sand and Gravel. The quarry, which cut through the interior of the top part of the volcano, provided basalt used in making roads.
Ranger Steve Edwards points across Round Top's crater, now tipped on its side, to Mt. Diablo in the distance. "That's not a volcano," he says, noting that many people think it might be because of its shape. "Here in Sibley, we're standing in the volcano."
Steve demonstrates the volcano's position in the past, and now. His hand is the volcano's caldera, the depression left in the top of a volcano after an eruption. Here, it's upright, as it was millions of years ago when Round Top was active.
The action begins; as tectonic plates in northern California slide past each other, the forces of their movements tilt the volcano.
Over the years, several mazes have been made at the bottom of the quarry, and are regular destinations for park visitors.
Watch your step and don't go beyond the fences or you may end up part of the quarry!